AU
C05D500

This DTC indicates an abnormal open-circuit condition in the Master Cylinder Solenoid Valve or its control circuit within the IPB (Integrated Power Brake) intelligent integrated braking system — Seal U

Braking System

This DTC indicates an abnormal open-circuit condition in the Master Cylinder Solenoid Valve or its control circuit within the IPB (Integrated Power Brake) intelligent integrated braking system.

In the BYD IPB system, the master cylinder pressure valve controls the hydraulic passage between the brake master cylinder and the wheel cylinders.

It serves as a key actuator for electro-hydraulic blended braking control, energy recovery, and ABS/ESC functions.

The ECU sets this DTC when it detects an open circuit in the valve drive circuit, abnormal valve position feedback, or a mismatch between the actual and commanded valve states lasting beyond the set threshold (typically exceeding 100ms).

This fault reduces brake pressure control accuracy and may trigger a brake system degraded mode (entering pure mechanical backup braking or limiting energy recovery).

This condition affects brake pedal feel and active safety functions (such as AEB and ACC).

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Open circuit in the IPB assembly master cylinder pressure solenoid valve coil, or valve mechanically stuck in the open position, causing the ECU to detect an open circuit.
  • 2Fault in the solenoid valve drive circuit on the internal IPB module PCB, such as a damaged MOSFET driver chip or cold solder joint.
  • 3Water ingress and corrosion in the IPB wiring harness connector (especially the 32-pin main plug), terminal back-out, or increased contact resistance (>1Ω)
  • 4IPB system power supply or ground circuit fault, causing insufficient solenoid valve drive voltage (below 10V)
  • 5Internal software fault in the IPB control unit (ECU) or ADC sampling circuit malfunction, causing a false valve status report.
What To Do
  • 1
    Connect the VDS2000 diagnostic tool, read the complete fault codes and freeze frame data, confirm the fault status (current/history), check for accompanying fault codes (such as other C05Dxx series pressure valve faults), and record the vehicle speed and brake pressure values at the time of the fault.
  • 2
    Perform a visual inspection: check the IPB assembly for brake fluid leaks and impact damage; check the wiring harness connector (located at the front left of the engine compartment) for looseness or water ingress; use a multimeter to measure the IPB supply voltage (constant power +B, standard 12-14V) and ground wire resistance (standard <0.5Ω).
  • 3
    Perform the IPB actuator test: Trigger the master cylinder pressure valve using the diagnostic tool. Listen for a crisp 'click' from the IPB (solenoid valve operating sound) and simultaneously use an oscilloscope to measure the PWM drive signal waveform at the ECU output terminal (normal: 12V square wave, frequency approx. 1kHz, variable duty cycle).
  • 4
    If the actuator test fails to respond, disconnect the IPB connector. Measure the master cylinder pressure valve coil resistance with a multimeter (standard value is typically 2-6 Ω; refer to the workshop manual for exact specifications). If the resistance is infinite or deviates from the standard value by ±20%, this indicates an open circuit in the internal IPB valve body coil.
  • 5
    If coil resistance is normal but the ECU has no drive output, check the solenoid valve drive circuit on the IPB internal PCB. Focus on the MOSFET and corresponding protection diode (requires professional electronic repair or replacement of the IPB assembly).
  • 6
    Check relevant wiring harness continuity: measure the resistance between the IPB connector and the valve body (should be <1Ω). Closely inspect the harness mounting points near the firewall for wear or damaged insulation, and check the connector for backed-out pins.
  • 7
    To replace the IPB assembly, execute the standard replacement procedure: disconnect the low-voltage battery negative terminal → remove the old IPB (collect brake fluid) → install the new IPB → execute the bleeding procedure using dedicated brake bleeding equipment → perform IPB ECU coding and calibration → perform a road test to verify braking function and energy recovery.
Real-World Cases
BYD DTC AI Analysis

Song Plus DM-i: ABS warning light on while driving with hard brake pedal

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Symptoms: ABS warning light illuminated during high-speed driving. Cluster displayed "Brake System Fault" and brake pedal felt stiff. Diagnosis revealed active DTC C05D500 with C003708 (pressure valve fault). Troubleshooting: Inspected IPB connector and found pin 15 (master cylinder pressure valve drive) slightly backed out. Reseating the connector cleared the fault, but it returned after 500 km. Replaced IPB wiring harness and applied conductive paste. Fault resolved. Root cause: Connector tolerance increased contact resistance. Excessive voltage drop during high-current drive triggered the fault codes.
BYD DTC AI Analysis

Brake system warning light stays on during new vehicle PDI.

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Symptoms: Pre-delivery inspection of a 2021 Song PLUS DMi showed multiple brake system warning lights on the dash. DTC C05D500 present, not clearable. Investigation: IPB supply voltage normal at 13.2V. During actuator testing, no sound from the master cylinder pressure valve. Disassembled the IPB and found a dry joint on the MOSFET (model IRFxxx) driving the valve on the internal PCB. Resoldered the connection; fault cleared. Root cause: Soldering process defect in the production batch caused an intermittent connection in the drive circuit.
BYD DTC AI Analysis

Brake system warning after driving through water

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Symptoms: After driving through standing water, the instrument cluster displayed "Brake System Fault." Fault codes C05D500 and U0121 (IPB communication fault) were present. Diagnosis: Inspected the IPB mounting location (left front of engine bay) and found water inside the connector. Pin A3 (valve drive power supply) was corroded and blackened. Cleaned and dried the connector; the fault temporarily cleared but returned after one week. Repair: Replaced the IPB assembly (suspected internal valve body corrosion) and the wiring harness connector. Fault cleared permanently. Root cause: The IPB mounts low in the engine bay. While driving through water, the connector seal failed and allowed water ingress, corroding the valve body drive circuit.
BYD DTC AI Analysis

IPB replaced without calibration, causing recurring DTCs.

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Symptoms: IPB assembly replaced due to brake noise. After approximately 50 km, DTC C05D500 returned with excessive brake pedal travel. Investigation: Only conventional brake bleeding performed after replacement. Technician did not use VDS2000 for 'IPB ECU coding' and 'pressure sensor zero-point calibration'. Repair: Performed complete calibration (master cylinder pressure sensor calibration and wheel cylinder pressure consistency learning). Fault cleared. Root cause: Skipping ECU software matching and sensor calibration caused incorrect control parameters for the master cylinder pressure valve.
Other Seal U Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.